1. Field of Invention
The present invention relates to drivers for DC-to-DC converters and more particularly to high voltage drivers with diode emulators.
2. Description of Related Art
Various DC-to-DC converters and related circuits often use two power MOSFETs, which are connected at an output node in a half-bridge configuration. The output node is connected to an inductor-capacitor filter. Such DC-to-DC converters allow the conversion of variable input battery voltages in a wide range, for example in the range of 6-24V to be stepped down to a lower constant output voltage. When the battery voltage decays from a nominal value of 24V to a value as low as 6V, the output voltage of the DC-to-DC converter remains essentially constant, because of the adaptive adjustments of the converter.
A converter includes a driver circuit, which drives the power MOSFETs. The driver circuit is controlled by a control circuit to turn on the high side power MOSFET and turn off the low side power MOSFET, and then turn on the low side power MOSFET and turn off the high side power MOSFET in an alternating manner. Thus, the control circuit, through the driver circuit, governs the charging and discharging intervals of the inductor-capacitor (LC) filter. The voltage, output by the LC filter, is then used to generate a step-down DC voltage.
In these DC-to-DC converters the high side driver cell, turning on or off the high side power MOSFET, operates at the high voltage set by the battery. Correspondingly, a high voltage has to be supplied to operate the high side driver cell.
In some converters, a bootstrap capacitor is used to supply the high voltage for the high side driver cell. This bootstrap capacitor is coupled between the node connecting the power MOSFETs and the high side driver cell. The bootstrap capacitor is charged up to a separately supplied voltage, for example, 5V, during the period, when the low side driver turns on the low side power MOSFET. Once the low side power MOSFET is turned off and the high side power MOSFET is turned on, the voltage at the node connecting the power MOSFETs flies up to close to the battery voltage, taking with it the voltage of the bottom plate of the bootstrap capacitor. Since the bootstrap capacitor has been charged to about 5V, the voltage of its top plate is then equal to the battery voltage plus 5V, thus well suited to supply the required high voltage for the high side driver cell.
Concerning related existing converter designs, U.S. Pat. No.: 4,908,551 discloses a half-bridge driver circuit, where the high side driver cell is powered by a bootstrap capacitor. However, the bootstrap capacitor is charged by an external high voltage supply. External discrete components increase fabrication cost and the complexity of the circuit. U.S. Pat. Nos. 5,373,435 and 5,502,632 each disclose a half-bridge driver circuit using an N-channel LDMOS transistor, as a bootstrap diode emulator. This circuit uses the drain of the LDMOS transistor to connect to the bootstrap capacitor and requires a substantial amount of extra circuitry to guarantee that the body voltage of the LDMOS transistor is properly biased with respect to the drain and source. Finally, U.S. Pat. No. 5,666,280 discloses a similar half-bridge driver circuit, but with a JFET structure being utilized as the bootstrap diode emulator. This circuit reduces the complexity of the body diode circuitry, however, it considerably increases the process complexity and expense with the introduction a high voltage JFET structure.